Remove attribute IMessage.deleted

We don't keep deleted messages.
Remove IConversation.last_updated
Fix persistence bug with BTrees

Subclasses of BTrees' ??BTree classes did not persist attributes. Move
the btree object to a .data attribute and add a mixin with the dict
interface that directly inherits Persistent.
Add plone.api
Add @@messaging-send and @@messaging-messages

Add a z3c.form for sending messages and an json view for getting a list
of messages
fix return result in @@messaging-messages, pep8

Return an object if there are no messages.

Lots of codeanalysis fixes
Add @@messaging-conversations view
Fire an IMessageSendEvent if a message is ... send
Fix indentation (pep8)
Rename mistyped message attribute
Add @@delete-message view
Add uid to messages response, extend test